Field of the Invention
The present invention relates to an image processing apparatus and an image processing method.
Description of the Related Art
Recently, high dynamic range (HDR) display, which is performed using multi-bit image data to obtain a display image (an image displayed on a screen) having a wider dynamic range than a conventional display image, is growing in popularity.
As a method of recording HDR image data (multi-bit image data) having a wide dynamic range, a method of dividing the HDR image data into base image data and brightness difference data and then recording the base image data and the brightness difference data has been proposed (Japanese Patent Application Publication No. 2011-193511). In other words, a format using base image data and brightness difference data has been proposed as a data format for HDR image data.
The base image data are image data constituted by a small number of bits, which are obtained by gradation-compressing (down-sampling) the HDR image data. The base image data are approximately 8-bit gradation image data, for example. A creator of image data determines a dynamic range of the base image data such that an important gradation range of the image data, within the wide dynamic range (a brightness range) of the HDR image data, is included in the dynamic range of the base image data.
The brightness difference data are data expressing a correspondence relationship between gradation values (brightness values) of the base image data and gradation values of the HDR image data.
The HDR image data can be obtained by converting the gradation values of the base image data into the gradation values of the HDR image data using the brightness difference data.
Various techniques for adjusting the dynamic range of image data in accordance with the content of the image data, user preferences, and so on have also been proposed.
For example, an image display apparatus that determines an average brightness level of input image data, lowers a brightness level of the input image data when the average brightness level is high, and raises the brightness level of the input image data when the average brightness level is low has been proposed (Japanese Patent Application Publication No. 2003-295837).
However, when the brightness level of HDR image data is modified using the technique described in Japanese Patent Application Publication No. 2003-295837, high gradation values of the HDR image data are limited to an upper limit value, and therefore blown-out highlights may appear on the display image. Furthermore, low gradation values of the HDR image data are limited to a lower limit value, and therefore blocked-up shadows may appear on the display image. As a result, a display image having a low contrast may be obtained.
Therefore, when the technique described in Japanese Patent Application Publication No. 2003-295837 is used, it may be impossible to obtain a display image that is pleasing to a user.
The problem described above occurs likewise when the dynamic range of the input image data is adjusted using an arbitrary adjustment value (a brightness range value) corresponding to a user operation.
Further, the user may wish to obtain image data having a dynamic range that is close to the dynamic range of the base image data. In the related art (including the technique disclosed in Japanese Patent Application Publication No. 2003-295837), however, the dynamic range of the HDR image data is adjusted using only the HDR image data as image data. Therefore, when the dynamic range of the HDR image data is adjusted according to the related art, image data having a dynamic range that is far removed from the dynamic range of the base image data are obtained. A case in which the important gradation range of the HDR image data is a gradation range on a high gradation side (a high gradation range) and the dynamic range of the base image data is a gradation range including the high gradation range, for example, may be envisaged. In this case, according to the related art, the high gradation range cannot be learned, and therefore the dynamic range of the HDR image data is adjusted to a gradation range not including the high gradation range.
With the related art, therefore, when the user wishes to obtain image data having a dynamic range that is close to the dynamic range of the base image data, image data having a dynamic range that is far removed from the dynamic range of the base image data are obtained instead.
As described above, according to the related art, the dynamic range of the input image data cannot be adjusted favorably.